Electro-hydraulic proportional actuator

ABSTRACT

An electro-hydraulic proportional actuator in which the actuator piston position is hydraulically controlled by fluid flow through solenoid valves operable by an electronic control system. An operator control provides an input voltage representing a desired piston position and a transducer generates a voltage representing the actual piston position. The difference between the input and output voltages generates a difference or error signal that is fed into a feedback system including a sample-and-hold circuit. A clock logic system is provided for transmitting the signal from the feedback system to the solenoid valves at predetermined intervals to activate either valve to move the actuator piston in either direction or to activate both valves to hold the piston at a desired position.

BACKGROUND OF THE INVENTION

Agricultural tractor draft controls traditionally have beenhydro-mechanical systems wherein, for example, implement draft is sensedmechanically. Through appropriate linkage, a hydraulic valve is actuatedto raise and lower the implement. Mechanical linkages have inherentlimited flexibility which has been further reduced by the implementationof environmentally isolated cabs.

The development of low cost, reliable electronic microcomputers hasintroduced an unlimited flexibility in the design of tractor controlsystems. For example, implement draft sensing need no longer be limitedto top-link, lower-link or output torque monitors, but can be correlatedwith wheel slip, engine speed, engine load and the like.

The primary requirement for implementing any tractor electro-hydraulicsystem is a low cost, contaminant insensitive, environmentally soundinterface device, or an actuator, to convert an electric signal toeither a mechanical or a hydraulic output.

SUMMARY OF THE INVENTION

The present invention relates to an electro-hydraulic proportionalactuator comprising two three-way solenoid operated valves to positionan actuator piston. Low pressure fluid is directed to either side of thepiston by activating either of the two solenoid valves. Hence, thepiston provides the necessary mechanical output at an amplified forcelimited only by the diameter of the piston.

The activation/deactivation of the solenoid valves is controlled by anelectronic control system. The system comprises an operator controlmeans that may be manipulated to provide an input voltage representing adesired actuator piston position. A transducer means coupled to theactuator piston generates an output voltage representing the actualposition of the piston. The difference between the input and outputvoltages generates a difference or error signal that represents thedifference between the actual and desired positions of the piston. Theelectronic control system includes a feedback system including asample-and-hold circuit in a feedback path having an input to receivethe difference or error signal. A clock logic means is provided fortransmitting the difference or error signal from the feedback system tothe solenoid valves at predetermined intervals to activate either valveto move the actuator piston in either direction or to activate bothvalves to hold the piston at a desired position.

DESCRIPTION OF THE DRAWINGS

Further features and advantages of the present invention will be mademore apparent as this description proceeds, reference being had to theaccompanying drawings, wherein:

FIG. 1 is a schematic diagram of an electro-hydraulic proportionalactuator adapted to be electronically controlled in accordance with thepresent invention;

FIG. 2 is a schematic of the electronic control circuit for theelectro-hydraulic proportional actuator.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the drawings, a hydraulic schematic diagram of theelectro-hydraulic proportional actuator, generally designated 10, isshown in FIG. 1. The actuator 10 has a double-acting, linearly movablepiston 11 contained in a cylinder 12 having at each end chambers 13 and14. A pair of three-way solenoid valves 15 and 16 communicate with thecylinder chambers 13 and 14, respectively. The solenoid valve 15 has avalve body 17 having an actuator port 18, an inlet port 19, and anoutlet port 21. The solenoid valve 16 has a valve body 22 having anactuator port 23, an inlet port 24, and an outlet port 25. The solenoids26 and 27 of the respective solenoid valves 15 and 16 control the flowof fluid through the valve bodies.

The valve inlet ports 19 and 24 are coupled through check valves 28 to afluid pressure supply line 29. The return or outlet ports 21 and 25 arecoupled to each other with the fluid flow therefrom being routed to asump 31 through a pressure compensated flow control valve 32.

In the neutral state, both the solenoid valves 15 and 16 are off. Theactuator piston 11 is free to float unless its position is determined byan external spring centering mechanism (not shown) or a spring (notshown) that biases the piston 11 to one end of its stroke. The neutralposition would be chosen so that it is the safe position for the systemif the electronic control system 33 for the actuator were to fail andpower to both solenoid valves were to go off. To move the actuatorpiston to the right, solenoid valve 15 is turned on and solenoid valve16 is turned off. To move the actuator piston to the left, solenoidvalve 15 is turned off and solenoid valve 16 is turned on. The speed ofthe actuator piston is fixed by the setting of the pressure compensatedflow control valve 32 in the return line. Any position of the actuatormay be held even in the presence of external loads in either directionby turning both solenoid valves on.

The decisions to move to the right, left or hold are performed by theelectronic control system 33 shown in FIG. 2.

The electronic control system 33 for operating the solenoid valves 15and 16 to direct fluid flow into the chambers 13 and 14 of the cylinder12 comprises an operator control 34 providing an input voltage V₁representing a desired actuator assembly piston 11 position. A positiontransducer 35 coupled to the piston 11 of the actuator 10 generates anoutput voltage Vv that represents the actual position of the piston 11.The difference between the input V₁ and transducer output voltage Vvgenerates a difference or error signal Ve₂ representing the spatialdifference between the actual and desired positions of the piston 11. Afeedback system, generally designated 36, includes a sample-and-holdcircuit 37 in the feedback path.

A clock logic means 38 transmits the difference signal Ve₂ from thefeedback system sample-and-hold circuit 37 to the solenoid valves 15 and16 at predetermined time intervals to activate either valve to move theactuator piston 11 in one direction or the other, or to activate bothvalves to hold the piston 11 at a desired position. The clock logicmeans 38 includes a clock 39 that provides a sampling frequency and iscoupled to one input 41 of an "OR" gate 42. The output 43 of the "OR"gate 42 is coupled to an input 44 of the sample-and-hold circuit 37 andthe signal from the "OR" gate 42 being denoted V_(sh).

The sample-and-hold circuit passes the Ve₂ signal when the V_(sh) signalis in a logic "1" state, i.e., when the clock 39 pulse is "high", andretains the last value of Ve₂ when the V_(sh) signal is in a logic "0"state, i.e., when the clock 39 pulse is in a low state, as in thefrequency diagram 45 shown in FIG. 2 adjacent the clock 39.

The clock logic means 38 includes a digital control 46 represented by alogic table, as follows:

    ______________________________________                                                               DIRECTION                                                           SOLENOIDS OF PISTON                                              V.sub.1 V.sub.e2   15      16    MOVEMENT                                     ______________________________________                                        |V.sub.1 | < V.sub.3                                                ALL        0       0     HOLD IN                                                                       NEUTRAL                                                                       POSITION                                     |V.sub.1 | > V.sub.3                                                |V.sub.e2 | < V.sub.4                                                  1       1     HOLD IN ANY                                                                   POSITION                                                                      OTHER THAN                                                                    NEUTRAL                                      |V.sub.1 | > V.sub.3                                                V.sub.e2 > V.sub.4                                                                       1       0     RIGHT                                        |V.sub.1 | > V.sub.3                                                V.sub.e2 < -V.sub.4                                                                      0       1     LEFT                                         ______________________________________                                    

For the solenoids: 1 is "On"; 0 is "Off"; V₃ is a preset thresholdvoltage determining the allowable deviation from the true desiredneutral position referred to hence as the neutral deadband; V₄ is apreset threshold voltage determining the allowable deviation from anytrue desired position other than neutral, referrred to hence as thedeadband outside neutral. "+" V₄ is an allowable deviation for movementof the piston 11 to the right as viewed in FIG. 1 and "-" V₄ is anallowable deviation for movement to the left.

One input 47 of the digital control 46 is coupled to the output of theoperator control 34 and receives the input voltage V₁. A second input 48is coupled to the sample-and-hold circuit 37 to receive the differenceor error signal Ve₂. The outputs 49 and 51 of the digital control arecoupled to the solenoids 15 and 16, respectively. The outputs 49 and 51are also coupled in parallel to the inputs 52 and 53 of an"EXCLUSIVE-OR" gate 54, the output 55 of which is coupled to the secondinput 56 of the "OR" gate 42.

The use of the sample-and-hold circuit enables the desired durabilityfor the solenoid valves to be obtained. The sampling circuit controlsthe mode of operation as follows:

A. Decides during the sampling instant to turn either solenoid "ON" or"OFF", moving the piston in either direction.

B. Holds when the desired position is reached until the next samplinginstant. The position is held by energizing both solenoids outside theneutral deadband and deenergizing both solenoids inside the neutralband.

The use of the sampling rate ensures the solenoids energize orde-energize at a rate no more than absolutely necessary for systemstability.

When the magnitude of the operator signal V₁, designated V₁ on the logictable, is less than the neutral deadband V₃, both solenoids 15 and 16are "off" and the piston remains in the neutral position regardless ofthe magnitude of Ve₂, designated Ve₂. When the magnitude of operatorsignal V₁ is greater than V₃, solenoid 15 or solenoid 16 is turned on tomove the piston to the right or left depending on the error signal Ve₂.If the magnitude of the error signal Ve₂ is greater than "+" Ve₄,solenoid 15 turns on and the piston moves to the right; if the magnitudeof the error signal Ve₂ is less than "-" V₄, then solenoid 16 turns onand the piston moves to the left. The movement of the piston continueseither to the right or left until Ve₂ is less than "+" or "-" V₄, as thecase may be, at which instant both solenoids are "on" and the pistonwill remain on "hold" at the desired position.

The "exclusive-or" gate functions in a manner such that an output fromit is available if one and only one of the solenoids is "on". If bothsolenoids are "on", that is, in the "hold" mode, the "exclusive-or" gatewill not provide an output. Hence, once the desired position is reachedand both solenoids are "on", no output from the sample-and-hold circuitcan occur until the next clock pulse. This ensures that the operation ofthe solenoids is only in conjunction with the clock frequency--not anyfaster. This results in an extended, useful life of the solenoids.

To summarize, the decisions to move left, right or hold are performed bythe electronic control system 33 shown in FIG. 2. The control systemuses a clock 39 to provide a sampling frequency when decisions are to bemade. During the sampling time interval, when the clock pulse is "high"and/or at a logic "1" state, the control system decides to turn eithersolenoid valve 15 or 16 on or off so as to move the piston 11 in eitherdirection or to hold. This decision is made by comparing the desiredposition as indicated by the input voltage V₁ with the actual positionof the actuator piston as measured by the position transducer (voltageVv) attached to the actuator piston. If the desired position is reached,that position is held until the next sampling instant occurs, even ifthe input continues to change. This ensures that the solenoids will beturned on and off at a maximum frequency determined by the clock. Thisensures an acceptable cycle life for the solenoid valves. The samplingfrequency depends on the maximum frequency that the actuator is designedto follow and can be made variable. For example, if the actuator isdesigned to follow a 5 Hz signal, the sampling rate could be set at 20Hz. The control logic could decrease the sampling frequency for inputsignals which are varying at a slower rate.

The positioning accuracy of the actuator, stated as a tolerance band TB,depends on the maximum time delay TD of the solenoid valves and thechosen rate of movement of the actuator piston V_(A).

    TB=V.sub.A ·TD.

In practice, the control logic can decrease the tolerance band to someextent by advancing the instant of actuation of the solenoid valves as afunction of the time delay of the valves and the rate of change of theinput.

The primary advantages of this electro-hydraulic actuator are:

1--A proportional output is achieved by using simple on/off three-waysolenoid valves which are relatively dirt insensitive and inexpensive.

2--The life of the solenoid valves is extended by limiting the maximumsampling frequency based on the maximum input frequency that is to befollowed.

3--The actuator may be designed to exert large forces over largedistances with little change to the basic control circuit.

It is to be understood that this invention is not limited to the exactconstruction illustrated and described above, but that various changesand modifications may be made without departing from the spirit andscope of the invention as defined in the appended claims.

We claim:
 1. An electro-hydraulic proportional actuator, comprising:an actuator assembly having a double-acting, linearly-movable piston contained in a cylinder having a chamber at each end; a pair of three-way solenoid valves each of which has an actuator port, an inlet port and an outlet port; each end chamber of the actuator assembly cylinder being coupled to the actuator port of one of the valves, and each valve inlet port being coupled through a check valve to a fluid pressure source and the return ports being coupled to each other with the fluid flow therefrom being routed to a flow control valve; and an electronic control system for operating the solenoid valves to direct fluid flow into and out of the cylinder chambers, comprising: an operator control means providing an input voltage representing a desired actuator assembly piston position, a transducer means coupled to the actuator assembly to generate an output voltage representing the actual position of the piston, the difference between the input and output voltages generating a difference signal representing the difference between the actual and desired positions of the piston, a feedback system including a sample-and-hold circuit in a feedback path having an input to receive the difference signal, and a clock logic means for transmitting the difference signal from the feedback system to the solenoid valves at predetermined intervals to activate either valve to move the actuator piston in either direction or to activate both valves to hold the piston at a desired position.
 2. An electro-hydraulic proportional actuator according to claim 1, in which:the electronic control system further comprises an "OR" gate having an output and two inputs; the clock logic means includes a clock means and a digital control means; the clock means provides a sampling frequency to one input of the "OR" gate; and the digital control means has an input coupled to the output of the sample-and-hold circuit and outputs coupled in parallel to the solenoid valves and to the second input of the "OR" gate; the output of the "OR" gate being coupled to an input of the sample-and-hold circuit.
 3. An electro-hydraulic proportional actuator according to claim 1, in which:the electronic control system further comprises an "OR" gate having an output and two inputs and an "EXCLUSIVE-OR" gate having an output and two inputs; the clock logic means includes a clock means and a digital control means; the clock means provides a sampling frequency to one input of the "OR" gate; and the digital control means has an input coupled to the output of the sample-and-hold circuit and outputs coupled in parallel to the solenoid valves and to the inputs of the "EXCLUSIVE-OR" gate; the output of the "EXCLUSIVE-OR" gate being coupled to the second input of the "OR" gate.
 4. An electro-hydraulic proportional actuator according to claim 2 or 3, in which:the flow control valve coupled to the outlet ports of the three-way solenoid valves limits the velocity of movement of the actuator piston.
 5. An electro-hydraulic proportional actuator comprising:an actuator assembly having a double-acting linearly movable piston contained in a cylinder having a chamber at each end; a pair of three-way solenoid valves each of which has an actuator port, an inlet port, and an outlet port; each end chamber of the actuator assembly cylinder being coupled to the actuator port of one of the valves; and each valve inlet port being coupled through a check valve to a fluid pressure source and the return ports being coupled to each other with the fluid flow therefrom being routed to a flow control valve; and an electronic control system for operating the solenoid valves to direct fluid flow into and out of the cylinder chambers, comprising; an operator controlled means for providing an input voltage V₁ representing a desired actuator assembly piston position, a transducer means coupled to the actuator assembly to generate an output voltage Vv representing the actual position of the piston, the difference between the input V₁ and output Vv voltages forming a difference signal Ve₂ representing the difference or error between the desired and actual positions of the actuator piston, a sample-and-hold circuit having an input to which the Ve₂ signal is coupled, a clock logic means including a clock means providing a sampling frequency coupled through first input of an "OR" gate having an output and two inputs to an input of the sample-and-hold circuit, the output of the "OR" gate comprising a sample-and-hold circuit, the output of the "OR" gate comprising a sample-and-hold circuit input control signal V_(sh), the sample-and-hold circuit passing the Ve₂ signal when the V_(sh) signal is in a logic "1" state and retaining the last value of Ve₂ when the V_(sh) returns to a logic "0" state, during a sampling instant, when the clock means sampling frequency pulse is high or in a logic "1" state, the control system operating to turn either solenoid valve on or off so as to move the piston in either direction or to hold the piston at a desired position until the next sampling instant.
 6. An electro-hydraulic proportional actuator according to claim 5, in which:the clock logic means includes a digital control means, one input of which is coupled to the output of the sample-and-hold circuit and two outputs of which are coupled in parallel to the solenoid valves and the inputs of an "EXCLUSIVE-OR" gate; the output of the "EXCLUSIVE-OR" gate being coupled to the second input of the "OR" gate; the "EXCLUSIVE-OR" gate ensuring the frequency of solenoid operation to be the same as clock frequency and no faster, whereby the life of the solenoids is extended especially at low frequency input signals generated by an operator.
 7. An electro-hydraulic proportional actuator according to claim 6, in which:the digital control means has a circuit corresponding to the following logic table:

    ______________________________________                                                                DIRECTION                                                            SOLENOIDS OF PISTON                                               V.sub.1 V.sub.e2   15      16    MOVEMENT                                      ______________________________________                                         |V.sub.1 | < V.sub.3                                                 ALL        0       0     HOLD IN                                                                        NEUTRAL                                                                        POSITION                                      |V.sub.1 | > V.sub.3                                                 |V.sub.e2 | < V.sub.4                                                   1       1     HOLD IN ANY                                                                    POSITION                                                                       OTHER THAN                                                                     NEUTRAL                                       |V.sub.1 | > V.sub.3                                                 V.sub.e2 > V.sub.4                                                                        1       0     RIGHT                                         |V.sub.1 | > V.sub.3                                                 V.sub.e2 < -V.sub.4                                                                       0       1     LEFT                                          ______________________________________                                    

in which for the solenoids: 1 is "On", 0 is "Off", V₃ sets the neutral deadband and V₄ sets the deadband outside neutral; the circuit producing digital output signals for activating the solenoids in accordance with the predetermined values of the neutral deadband and the deadband outside of the neutral zone and in conjunction with the operator controlled input position values.
 8. An electro-hydraulic proportional actuator according to claims 5 or 7, in which:the flow control valve coupled to the outlet ports of the solenoid valves limits the velocity of movement of the actuator piston. 